how to sum symbolic equation

Dear all,

I would like to sum a symbolic function as follow:

J = sigma_j=1 ^J=5 (X{j+1} - X{j})

at the end It should answer should be as follow:

(X2 - X1) + (X3 - X2) + (X4 - X3) + (X5 - X4)

I tried with symsum but it didn't work.

any help would be greatly appreciated.

Answer 1

[Walter Roberson]

X = sym('X', [1,N]);
J = X(end) - X(1);

More generally,

syms f(x)
J = sum(arrayfun(f, x(2:end)-x(1:end-1)))

In earlier MATLAB you might need

temp = arrayfun(f, x(2:end) - x(1:end-1), 'uniform, 0);
J = sum([temp{:}]);

Notice the complete lack of symsum. symsum can never be used to index variables: you need to form the list of definite elements and sum them. symsum is for finding theoretical formula such as symsum(k) being k*(k+1)/2 not for adding definite terms.